Downy mildew is a disease caused by oomycetes, water moulds rather than true fungi. On cannabis it has historically been attributed to Pseudoperonospora cannabina; the hop downy mildew pathogen, Pseudoperonospora humuli, has also been reported on hempbeing reviewed. Infected leaves show pale yellow, angular patches on the upper surface and greyish to purplish downy growth underneath, where spores form.
In practice
Downy mildew needs free water and cool temperatures, so it is chiefly an outdoor and greenhouse problem in wet spells, not a common indoor one. Spores spread by wind and splash, and infected leaves curl, brown and drop. Growers manage it with airflow, wider spacing, watering at the base, removing infected leaves and keeping hop yards and cannabis apart where both are grown. Diagnosis is easier with a hand lens on the leaf underside early in the morning, when sporulation is visible. Check what is registered for cannabis locally.
Don't confuse with
- Powdery mildew: white powder on upper surfaces, grows without leaf wetness.
- Septoria leaf spot: tan lesions with dark margins and pycnidia.
- Magnesium deficiency: interveinal yellowing with no growth underneath.
